Add documentation about opensips-mi task

docs/tasks/opensips-mi.md

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+# SIPssert Testing Framework OpenSIPS MI Task
+
+Task used to run a MI command using the [OpenSIPSMI Python module](https://github.com/OpenSIPS/python-opensips). It is designed to be more lightweight than its alternative, the [OpenSIPS CLI](./opensips-cli.md) task, stripping down the unnecessary features and focusing on the MI commands.
+
+## Behavior
+
+The task is able to communicate with one or more OpenSIPS instances using the
+[OpenSIPSMI Python module](https://github.com/OpenSIPS/python-opensips/blob/main/docs/mi.md) over HTTP, Datagrams or FIFO files. It has two different working modes: running a single command, or
+running a script, either with the `.sh` (executed with bash), either a `.py`
+file (executed with Python).
+
+## Defaults
+
+The variables overwritten by default by the task are:
+
+* `image`: default image to run is `opensips/python-opensips`
+
+## Settings
+
+Additional settings that can be passed to the task:
+
+* `script`: optional, a path to a `.sh` or `.py` script that can be executed;
+if missing, the `opensips-mi` tool will execute the command that is passed through the `args` parameter (see the example below)
+* `mi_ip`: optional, the IP to the OpenSIPS MI listener; if missing,
+`127.0.0.1` is used
+* `mi_port`: optional, the port to the OpenSIPS MI listener; if missing,
+`8888` is used
+* `mi_type`: optional, the type of the MI connection; if missing, `http` is used
+
+## Examples
+
+```
+ - name: Reset Statistics
+   type: opensips-mi
+   args: reset_all_statistics
+```
+
+```
+ - name: Check Domain
+   type: opensips-mi
+   script: check_domain.py
+   args: domain.sip
+
+   # check_domain.py is a Python script that uses the OpenSIPSMI module to execute the MI command `domain_dump` and search for `domain.sip`
+```